1. Field of the Invention
The present invention relates to a three-phase, three-wire active power filter. More particularly, the present invention relates to the active power filter applied to a three-phase, three-wire power system for filtering harmonic currents generated therein.
2. Description of the Related Art
The power electronic devices with high-voltage rated, high-current rated, and high switching speed characteristics have been developed due to the improvement of semiconductor manufacturing technique, recently. Power electronic devices are applied in electric power facilities, such as switching power supply, uninterruptible power supply, motor driver, arc furnace, trolley car, battery charger, and lighting appliance etc. The electric power facilities may generate a large amount of harmonic current due to the nonlinear input characteristic of such loads. The harmonic current will pollute the power system and result in the problems such as transformer overheat, rotary machine vibration, degrading voltage quality, electric power components destruction, medical facilities malfunction etc.
In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC 61000-3-5, and IEC 61000-3-4 etc., have been established by international research institute. In Taiwan, there is also established a power control provisional standard for harmonic by Taiwan Power Company. Therefore, how to solve the harmonic problems is an important topic in today's power system worldwide.
Conventionally, the passive power filter is used to solve the problems of harmonic. The passive power filter generally consists of inductors and ac capacitors. However, the passive power filter causes the problems of resonance and neighboring harmonic current injection that may damage the passive power filter. Additionally, it is quite hard to obtain a better filtering effect of the passive power filter due to system reactance that may easily affect the filtering characteristic of the passive power filter.
Many other solutions employing an active power filter for solving the harmonic problem are disclosed in the well-known art, such as U.S. Pat. Nos. 6,472,775, 5,977,660 and 5,321,598.
Referring initially to FIG. 1, a schematic circuitry of a conventional active power filter is illustrated. In the illustrated circuitry, a power source 1 supplies three-phase electric power to a load 3 with which to parallel-connect an active power filter 4. The active power filter 4 includes a dc capacitor 40, a power converter 41 and a switching ripple filter 42. The power converter 41 is employed to switch a voltage of the dc capacitor 40 that creates three-phase compensating currents via the switching ripple filter 42 to be injected into a power line. The power converter 41 is a power electronic switch set having a three-arm bridge structure. The switching ripple filter 42 is selected from an inductor or a combination of an inductor and a capacity which is used to filter high-frequency ripple currents due to switching power electronic switches of the power converter 41. Although the active power filter 4 performs a better filtering characteristic, the capacity of the power converter 41 of the active power filter 4 must be greater than the product of the harmonic current of the load 3 and the voltage of the power source 1. Therefore, the capacity and manufacture cost of the power converter 41 of the active power filter 4 must be significantly increased so that applications of the active power filter 4 are disadvantageously limited.
A hybrid power filter consisting of a passive power filter and a power converter is developed to overcome the high-capacity and high-price problems caused by the power converter of the active power filter because the passive power filter is used to lower the capacity of the power converter. For example, the hybrid power filter is disclosed in U.S. Pat. Nos. 5,567,994, 5,731,965 and 6,717,465. However, these hybrid power filters have been applied to a three-phase, three-wire power system which constantly employs a power electronic switch set having a three-arm bridge structure. In the three-arm bridge structure, at least six power electronic switches must be given if each arm is configured a pair of power electronic switches.
However, there is a need for reducing the number of the power electronic switches for the power converter that may reduce manufacture cost of the hybrid power filter for increasing its market competitiveness.
The present invention intends to provide a three-phase, three-wire active power filter including a reactive power compensating capacitor and a power converter. The power converter only employs a power electronic switch set with a two-arm bridge structure and permits one of power lines of a three-phase, three-wire power system to be connected to either a positive or negative dc terminal of the power electronic switch set without passing through a power electronic switch. Consequently, the number of the power electronic switches employed in the three-phase, three-wire active power filter can be reduced.